<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
       .round1{
        width: 20px;
        height:20px;
        border-radius: 10px;
        border: 1px solid gray;
       }
       .round::after{
        display: block;
        content: '√';
        clear: both;
       } 
    </style>
</head>
<body>
    <div id="app">
      <div @click="active=1" :class="{[aa]:active===1}" class="round1"></div>
      <div @click="active=2" :class="{[aa]:active===2}" class="round1"></div>
      <div @click="active=3" :class="{[aa]:active===3}" class="round1"></div>  
    </div>
</body>
<script src="./vue.js"></script>
<script>
    let{createApp,ref}=Vue;
    createApp({
        setup(){
           let aa=ref('round')       
           let active=ref(3)
            return{
               aa,
               active
            }
        }
    }).mount("#app")
</script>
</html>